It's not hard to imagine international tensions bubbling over as the planet heats up, and defence planners have begun to pay more attention to the risk of climate-related conflicts.

One landmark assessment prepared for the Pentagon in 2003 considered a specific scenario for abrupt climate change in which conflicts over land and water use grow through the century. If the planet's ability to support its growing population were to drop abruptly, "humanity would revert to its norm of constant battles for diminishing resources", the report warned. The message was underscored in 2007 with another report in which a group of retired generals and admirals concluded that "projected climate change poses a serious threat to America's national security".

However, the issues are very complex. Scholars have long tussled over the question of how the environment intersects with social factors to breed war. Drought, in particular, is a perennial marker of conflict but the connection isn't a simple one. The horrific violence in Darfur, Sudan, for example, followed decades of strained relations between nomadic herders and farmers, coinciding with a sustained drop in rainfall from the 1970s onward. However, the dramatic onset of warfare in 2003 doesn't appear to be directly linked to short-term dryness.

As the world's subtropics heat up and dry out, many important regions – including parts of the southwest US and northern Mexico, and the nations ringing the Mediterranean – will see an intensifying risk of major drought, which may feed other sources of societal stress and international conflict. These same parts of the world could also be well suited for hosting mammoth solar farms to send energy across international lines – another potential wrinkle in the geopolitics of tomorrow.